The present invention relates to novel silicon oxynitride glass compositions containing substantial amounts of magnesium oxide, and to the production of glass-ceramic articles therefrom.
Predominantly oxide glasses and glass-ceramics containing structural nitrogen, that is nitrogen bound up in the structure of the glass or glass-ceramic rather than present merely as gaseous inclusions, are known. In U.S. Pat. No. 3,582,307 a procedure resulting in the incorporation of a small amount of nitrogen in an alkali boroaluminosilicate glass was described, that procedure involving bubbling nitrogen through the molten glass under controlled atmosphere conditions. In U.S. Pat. No. 4,186,021, and in related patents referred to therein, thermally crystallizable glasses containing substantial nitrogen concentrations are disclosed.
The glasses of the latter patent can be converted to glass-ceramics comprising such nitrogen-containing crystal phases as nitrogen-mullite (Al.sub.3 Si.sub.2 O.sub.7 N), silicon oxynitride (Si.sub.2 ON.sub.2), and beta-silicon nitride (.beta.-Si.sub.3 N.sub.4) solid solution as well as crystal phases normally observed as silicates but in this case containing small amounts of structural nitrogen. Some of these nitrogen-containing crystal phases had previously been observed in the course of research into conventional nitride ceramics, but not in thermally crystallizable glasses.
U.S. Pat. No. 2,920,971 provides the earliest disclosure in the field of glass-ceramics. That patent describes the manufacture of a predominantly crystalline article (a glass-ceramic) through the heat treatment of a glass article fabricated from a thermally crystallizable glass. The crystallizable nature of the glass derives from the inclusion therein of a small amount of a nucleating agent which promotes crystal nucleation and growth in dense, homogeneous fashion throughout the volume of the glass during heat treatment.
Following this initial disclosure, the bulk of experimental work in the glass-ceramic field has involved the development of new thermally crystallizable glasses, formable into glass articles by conventional glass-forming techniques but thereafter crystallizable in situ by an appropriate heat treatment to glass-ceramics containing new crystal phases and exhibiting new properties. However, most of this work has been concentrated in oxide glass-forming systems.